The present invention relates to direct current torque motors of the type intended for forward or reverse rotation by an amount less than one complete revolution of the motor rotor. Torque motors of this type typically are employed for servo actuator devices where fractional revolution rotation of a shaft is required for performing a work function. In particular, it has been desired to utilize torque motors for actuating the air intake throttle valve of an internal combustion engine in response to a control signal provided by an electronic controller. This is due in part to the recently imposed strict engine exhaust emission requirements for motor vehicles which have necessitated electronic control of vehicle engine operating parameters.
In such internal combustion engine throttle applications and particularly engine throttle applications for motor vehicles, it is desired to minimize the size and weight of the torque motor in as much as it must be attached to the engine throttle body structure and is therefore subjected to the elevated temperature and vibration encountered by the engine structure. Furthermore, the torque motor must provide a substantial torque output with minimum motor coil excitation current in as much as the motor is operating, in a typical automotive application at relatively low voltages on the order 12-24 volts DC. Therefore, it has been desired for an electrically operated motor vehicle engine throttle actuator to provide a torque motor which has a maximum torque with a minimum of magnetic pole structure mass in the rotor and stator and which is robust and accurate in rotor positioning for a given coil excitation current in order to provide accurate positioning of the vehicle throttle in response to an electrical throttle control signal from an onboard electrical controller. Furthermore, it has been desired to provide a low voltage direct current torque motor of minimum mass and size for a vehicle throttle application which does not require pole structure fabrication of relatively exotic or expensive materials of high magnetic permeability but is capable of being fabricated from relatively low cost iron based material.
Heretofore, it has been found possible to provide a direct current torque motor for vehicle engine throttle operation which provides the requisite torque for insuring proper throttle positioning for a given control signal, however, the known torque motor designs have proven either prohibitive in production costs for high volume applications, or have been prohibitively bulky and heavy.
Therefore, it has long been desired to provide a direct current torque motor for servo actuator operation at relatively low voltages, particularly for vehicle engine throttle positioning which has a minimum mass, volume and maximizes the torque output therefrom for a given motor coil excitation current and which does not require relatively high cost materials.